Fail queries fast if no DNS servers are configured.

When no DNS servers are configured (and thus there is no chance
that the DNS query will suceed), res_nsend returns early, but
it does not tell the cache that the query has failed.
Therefore, if the caller retries the query, it will block for
PENDING_REQUEST_TIMEOUT (= 20 seconds) waiting for the "existing
query" (which isn't actually doing anything) to complete.

Bug: 18240188
Bug: 18327075
Change-Id: I0df13ff4a17ee65e640be96695a3af31b020963a